“In Feb. 2024, Stardew Valley reached 30 million copies sold, and if we assume each copy sold for $15, that means that the game could have generated a revenue of $450 million. A modest 10 percent profit margin puts ConcernedApe’s earnings at $45 million, a number that is likely to increase in the future.” Source: https://dotesports.com/stardew-valley/news/how-much-money-di...
Why would 1 unit of this game have a "profit margin" of 10%? It's a video game. He's not selling canned goods.
There are regional pricing and sales. Good chunk of those 30 millions sold way below US price. He certainly earned what you say though.
TBH the profit margin on this game is probably closer to 100% than 10%, it was a solo-dev game so never much overhead, I think one guy was hired to work on it.